One of the characteristics of tumor cells (cancer cells) is that as compared to normal cells, they undergo rapid cell division, resulting in a significantly high cell proliferation rate. Thus, as anticancer agents to kill tumor cells or to inhibit their proliferation, drugs that inhibit division of tumor cells (cancer cells) are used. For example, alkylating agents are drugs that act on the DNA of tumor cells (cancer cells) to crosslink the bases between double strands, thereby inhibiting DNA replication and blocking cell division. Antimetabolites such as 5-FU, etc., are drugs that inhibit DNA synthesis and block cell division.